African C3 grass n-alkane distributions & plant chemotaxonomy

Reconstructing past vegetation can elucidate the timing, climate forcings, and biotic mechanisms of ecosystem change. Plant macro- and microfossils are traditionally used to study past vegetation but suffer from production and taphonomic biases, such as underrepresentation of important herbaceous vegetation components. Geochemical proxies can fill this gap, but carbon isotopes (δ13C) in isolation are unable to distinguish between structurally different C3 habitats, such as forests and grasslands. Thus, new geochemical methods to identify grassy C3 ecosystems are necessary. We present n-alkane chain length distributions of 209 plant specimens from two Kenyan C3-dominated ecosystems, representing a wide range of plant functional types (PFTs). We find that C3 PACMAD grasses produce exceptionally high abundances of long chain C33 and C35 n-alkanes (ACL = 32.7, mean C33 + C35 relative abundance = 0.69), unlike other C3 PFTs which produce low abundances of C33 and C35 (ACL = 28.9 to 30.3, mean C33 + C35 relative abundance = 0.0 to 0.21). These data demonstrate that n-alkane distributions can serve as a proxy for some African C3 PACMAD grasses, offering a new paleoecological tool for distinguishing C3 vegetation types.